1. Field of the Invention
This invention relates to an electrode for secondary cells comprising a carbon material as an active substance for the electrode, and a method for making an electrode of the type mentioned above. The invention also relates to a nonaqueous electrolyte secondary cell comprising the electrode.
2. Description of the Prior Art
Recently, electrodes made of a carbon material as an active substance therefor have been frequently employed in order to obtain secondary cells having a high capacity and good charge and discharge cycle characteristics. For obtaining secondary cells having a higher capacity and better charge and discharge cycle characteristics, extensive studies have been made on carbon electrodes which have a high electrode capacity and good charge and discharge cycle characteristics.
For instance, Japanese Laid-open Patent Application No. 5-78909 proposes an electrode which is obtained by forming, on a porous ceramic substrate, a carbonaceous material which has a structure slightly disarranged on comparison with highly oriented carbon having a graphite crystal structure and also has a selective orientation. A secondary cell using this electrode exhibits a high capacity and good charge and discharge cycle characteristics. However, such a carbonaceous material can be formed only in the form of a thin film when using a chemical deposition (CVD) method, thus making it difficult to obtain the carbonaceous material in amounts sufficient for practical applications.
Japanese Laid-open Patent Application No. 5-94838 proposes a composite carbon material having a composite structure of the core and the skin layer, which is obtained by forming a carbonaceous skin layer having a specific value determined by the Raman spectroscopy on individual particulate or fibrous carbon cores according to a CVD method. However, it is difficult to obtain the composite carbon martial, wherein each core is fully covered with the carbonaceous material, within a short time and in an efficient manner. Thus, a difficulty is involved in industrial mass-production of an electrode by use of the composite carbon material.
In fact, we have made a follow-up test wherein while particulate or fibrous carbon cores are invariably fluidized by use of an agitated, fluidized bed in a rotary kiln, the skin layer of a carbonaceous material is formed on individual cores. As a result, it has been found that it takes a long time of several tens of hours before the skin layer of a carbonaceous material with a uniform thickness is formed. In addition, during the course of the formation of the skin layer, carbon bodies mutually coagulate, thereby forming coagulms having a larger size.
Where the larger-sized coagulms are divided into finer particles so as to utilize them as an electrode active substance, the carbon bodies may be broken at the core portion thereof, or the skin layer may be separated to expose the core thereat. Such exposed cores undesirably facilitate reaction with an electrolyte during charge and discharge cycles, so that the electrolyte is very likely to decompose. The decomposition of the electrolyte leads to the lowering of a charge and discharge repetition life (i.e. a charge and discharge cycle characteristic).
In order to shorten the formation time and to prevent the coagulation of the carbon bodies, the flow rate of a hydrocarbon gas is increased to allow good fluidization. As a result, it has been found that some of the carbon bodies are lost by scattering, thus resulting in the lowering of yield.